1. Field of the Invention
This invention relates to an optical pickup device for writing an information signal to and/or reading an information signal from an optical recording medium, to an optical disc apparatus provided with such an optical pickup device and adapted to recording an information signal on and reproducing an information signal from an optical recording medium and also to a method of detecting a track discrimination signal for the purpose of detecting the position of a target recording track.
2. Prior Art
Various types of optical recording medium such as optical discs have been proposed along with optical disc apparatus adapted to recording an information signal on and reproducing an information signal from such an optical recording medium. An optical disc is typically used as optical recording medium with such an optical disc apparatus, although the system of using an optical disc may vary. An optical pickup device is typically used for an optical disc apparatus when writing an information signal to and reading an information signal from an optical disc.
An optical pickup device is normally provided with a light source such as a semiconductor laser and the flux of light emitted from the light source is converged and irradiated onto the signal recording surface of an optical disc by way of an objective lens. Such an optical pickup device is so configured that an information signal is written onto the signal recording surface by means of a flux of light irradiating the signal recording surface and an information signal is read out from the signal recording surface by detecting the flux of light irradiated onto and reflected by the signal recording surface.
The optical pickup device is designed to write or read information signals along the land sections or the groove sections formed helically or coaxially on the signal recording surface of the optical disc.
Meanwhile, efforts have been made to densely record information signals on an optical disc. For instance, a xe2x80x9cDVDxe2x80x9d (tradename) having a recording capacity of 4.7 GB which is about seven times as much as that of a xe2x80x9cCompact Disc (CD)xe2x80x9d (tradename), or 650 MB, is known, although the both discs have a same diameter of 120 mm.
The trend toward a higher recording density is also remarkable in the field of xe2x80x9crewritable discsxe2x80x9d where information signals can be recorded and reproduced. Optical disc apparatus adapted to use a so-called xe2x80x9cDVD-RAMxe2x80x9d or a rewritable xe2x80x9cDVDxe2x80x9d have also been proposed. xe2x80x9cDVD-RAMsxe2x80x9d are used with a xe2x80x9cland/groove recording systemxe2x80x9d designed to record information signals not only on the land sections or the groove sections but on both the land sections and the groove sections of the disc in order to densely record signals there.
However, rewritable high recording density discs using the xe2x80x9cland/groove recording systemxe2x80x9d such as xe2x80x9cDVD-RAMsxe2x80x9d are accompanied by the following problems because both the land sections and the groove sections have a substantially same width.
In the case of an optical disc to be used with a xe2x80x9cland recording systemxe2x80x9d for recording information signals only on the land sections that have a width greater than the groove sections, the phase of the tracking error signal (TE) and that of the sum signal (SUM) using a return beam (or a main spot when using a xe2x80x9cthree-spot methodxe2x80x9d) is shifted by a xe2x80x9cxc2xcxe2x80x9d period from each other as shown in FIG. 1 of the accompanying drawings, where a period refers to the time required to move from the head of a groove section to that of the next groove section.
Therefore, for tracking control of trying to reduce the tacking error signal (TE) to nil, there are two occasions that can make the tracking error signal (TE) equal to zero including one where the flux of light is irradiating a land section and one where the flux of light is irradiating a groove section and these two occasions can be discriminated by seeing the level of the sum signal (SUM).
The signal for discriminating the occasion where the flux of light is irradiating a land section and the occasion where the flux of light is irradiating a groove section is referred to as xe2x80x9ctrack discrimination signalxe2x80x9d or xe2x80x9ccross track signal (CTS)xe2x80x9d. When the xe2x80x9cland recording systemxe2x80x9d is used and the level of the sum signal (SUM) differs remarkably between when the flux of light is irradiating a land section and when it is irradiating a groove section, it is possible to use the AC component of the sum signal (AC-SUM) for the xe2x80x9ctrack discrimination signalxe2x80x9d as shown in FIG. 2 of the accompanying drawings. Referring to FIG. 2, the AC component of the sum signal is a track discrimination signal whose phase is shifted by 90 degrees from that of the tracking error signal.
Therefore, when the xe2x80x9cland recording systemxe2x80x9d is used, it is possible to accurately find out the direction in which the light spot is moved and the number of tracks by which the light spot is shifted relative to the track to be used for the recording operation by using both the tracking error signal and the signal of the AC component of the sum signal even in a high speed seek operation. Thus, the operation of counting the number of tracks to be crossed and that of drawing the tracking servo can be conducted reliably.
However, on the other hand, when the xe2x80x9cland/groove recording systemxe2x80x9d is used, both the land sections and the groove sections are normally made to have a substantially same width in order to optimize the recording/reproduction performance of the optical disc apparatus. As a result, the level of the above described sum signal (SUM) does not differ noticeably between when the flux of light is irradiating a land section and when it is irradiating a groove section as shown in FIG. 3 of the accompanying drawings so that it is no longer possible to generate a track discrimination signal from the sum signal.
Then, consequently it is highly difficult to access a given track instantaneously in a high speed seek operation that frequently takes place when using an external memory or a video recording/editing apparatus for professional applications and therefore the problem of a long access time occurs.
In view of the above identified circumstances, it is therefore the object of the present invention to provide an optical pickup device that can generate a track discrimination signal without requiring the use of a large number of parts or a complex device configuration even when an optical recording medium adapted to the xe2x80x9cland/groove recording systemxe2x80x9d such as a xe2x80x9cDVD-RAMxe2x80x9d is used and a method of detecting a track discrimination signal for such an optical pickup device as well as an optical disc apparatus adapted to comprising such an optical pickup device and use such a method of detecting a track discrimination signal.
According to the invention, the above object is achieved by providing an optical pickup device comprising:
a light source having at least a spot for emitting a flux of light;
an objective lens for converging the flux of light on the signal recording surface of an optical recording medium and irradiating it with the flux of light; and
a photo-detection means for receiving the flux of light reflected from the signal recording surface of the optical recording medium;
the flux of light emitted from the light source producing a main flux of light for forming a main spot on the signal recording surface of the optical recording medium for the purpose of recording and/or reproducing an information signal and an auxiliary flux of light for forming an auxiliary spot on the signal recording surface of the optical recording medium at a position separated from the main spot;
the auxiliary flux of light travelling a distance between the objective lens and the focal point different from the main flux of light;
the auxiliary spot being formed at a position expressed by
S≈Pn/2, 
S being the absolute value of the distance relative to the main spot in the normal direction of the recording tracks formed on the signal recording surface relative to the man spot,
P being the track pitch,
n being an integer.
For the purpose of the invention, the expression xe2x80x9ctrack pitch (P)xe2x80x9d refers to xe2x80x9cthe distance between a land section and the next land sectionxe2x80x9d (or xe2x80x9cthe distance between a groove section and the next groove sectionxe2x80x9d) even in the case of recording information signals both on the land sections and the groove sections on an optical disc by using the xe2x80x9cland/groove systemxe2x80x9d.
In another aspect of the invention, there is provided an optical disc apparatus comprising:
a rotary drive mechanism adapted to rotate an optical recording medium removably fitted thereto and having land sections and groove sections, both being capable of recording information signals;
an optical pickup device fitted to the rotary drive mechanism in order to write an information signal to and reading an information signal from the optical recording medium being rotated by the rotary drive mechanism;
a signal processing circuit for generating a replay signal on the basis of the output signal of the optical pickup device; and
a servo circuit for controlling the position of the objective lens of the optical pickup device on the basis of the output signal of the optical pickup device;
the optical pickup device including:
a light source having at least a spot for emitting a flux of light;
an objective lens for converging the flux of light on the signal recording surface of an optical recording medium and irradiating it with the flux of light; and
a photo-detection means for receiving the flux of light reflected from the signal recording surface of the optical recording medium;
the flux of light emitted from the light source producing a main flux of light for forming a main spot on the signal recording surface of the optical recording medium for the purpose of recording and/or reproducing an information signal and an auxiliary flux of light for forming an auxiliary spot on the signal recording surface of the optical recording medium at a position separated from the main spot;
the auxiliary flux of light travelling a distance between the objective lens and the focal point different from the main flux of light;
the auxiliary spot being formed at a position expressed by
S≈Pn/2, 
S being the absolute value of the distance relative to the main spot in the normal direction of the recording tracks formed on the signal recording surface relative to the man spot,
P being the track pitch,
n being an integer.
In still another aspect of the invention, there is provided a method of detecting a track discrimination signal in an operation of reading an information signal from an optical recording medium removably fitted to a rotary drive mechanism adapted to rotate the optical recording medium and having land sections and groove sections, both being capable of recording information signals, by means of an optical pickup device;
said optical pickup device comprising:
a light source having at least a spot for emitting a flux of light;
an objective lens for converging the flux of light on the signal recording surface of an optical recording medium and irradiating it with the flux of light; and
a photo-detection means for receiving the flux of light reflected from the signal recording surface of the optical recording medium;
the flux of light emitted from the light source producing a main flux of light for forming a main spot on the signal recording surface of the optical recording medium for the purpose of recording and/or reproducing an information signal and an auxiliary flux of light for forming an auxiliary spot on the signal recording surface of the optical recording medium at a position separated from the main spot;
the auxiliary flux of light travelling a distance between the objective lens and the focal point different from the main flux of light;
the auxiliary spot being formed at a position expressed by
S≈Pn/2, 
S being the absolute value of the distance relative to the main spot in the normal direction of the recording tracks formed on the signal recording surface relative to the man spot,
P being the track pitch,
n being an integer;
the track discrimination signal being generated on the basis of the output signal of the photo-detection means.
Thus, with an optical pickup device, an optical disc apparatus and a method of detecting a track discrimination signal according to the invention, it is possible to reliably obtain track discrimination signal even when recording a signal for xe2x80x9cland/groove recordingxe2x80x9d, using a xe2x80x9cland/groove recording mediumxe2x80x9d.
Therefore, conventional control methods such as those using the operation of counting the number of recording tracks to be crossed at seek and that of drawing the tracking servo can be applied reliably to xe2x80x9cland/groove recordingxe2x80x9d.
Thus, according to the invention, it is now possible to provide an optical pickup device and an optical disc apparatus adapted to the xe2x80x9cland/groove recording systemxe2x80x9d that typically uses a xe2x80x9cDVD-RAMxe2x80x9d and realized by using a limited number of parts and a simple device configuration even when an optical recording medium.
Additionally, according to the invention, it is possible to provide an optical pickup device and an optical disc apparatus that can be used not only with xe2x80x9cDVD-RAMsxe2x80x9d but also with various optical discs conforming to different standards for xe2x80x9cland/groove recordingxe2x80x9d and realized by using a limited number of parts and a simple device configuration.